When a power-tube screen "shorts"....

[pula58]

Is the screen grid shorting to the cathode?
Seems like it must be. If it was shorting to the grid, then the grid leakage resistors are large enough in value to limit the current to a degree that the screen grid resistors would not fry.

So then, my feeling is that fusing in between the power tube cathodes and ground (for a fixed bias arrangement) would protect against damage from this type of failure.

Similarly, can the plate short to the cathode? And if so, would not the cathode fusing protect against damage to the OT and power supply?

Your thoughts on this?

Thanks!

[Stevem]

You can never fully protect from a screen grid failure as sometimes they just snap off due to vibration.
The main concern as I see it is to protect the OT from excess current so a fuse in its center tap should do that.

[pula58]

Hi Steve, thanks for your input.
I guess what I am trying to first understand is exactly what happens when there is a screen short. Where is the screen shorting-to?

I like the idea of cathode fuses since the voltages are low there.

[tubeswell]

Signal grid and suppressor grid* first, then cathode (but could be plate as well, although that is not likely to cause meltdown), depending on how the screen grid (and the screen grid support rods) start to melt and flex inside the electrode cage.

* or beam plate (in a tetrode)

[pula58]

Signal grid and suppressor grid* first, then cathode (but could be plate as well, although that is not likely to cause meltdown), depending on how the screen grid (and the screen grid support rods) start to melt and flex inside the electrode cage.

* or beam plate (in a tetrode)

Seems like a cathode fusing scheme would protect amp from damage... no?

[labb]

The Crate Blue Voodoo fuses the cathode. Take a look a BV-60 schematic for the power tubes

[pula58]

The Crate Blue Voodoo fuses the cathode. Take a look a BV-60 schematic for the power tubes

Ah...indeed it does...thanks P.

[tubeswell]

Signal grid and suppressor grid* first, then cathode (but could be plate as well, although that is not likely to cause meltdown), depending on how the screen grid (and the screen grid support rods) start to melt and flex inside the electrode cage.

* or beam plate (in a tetrode)

Seems like a cathode fusing scheme would protect amp from damage... no?

You could simply use a fusible cathode resistor rated at just above maximum tube current draw

[trobbins]

A fuse is a better defined protection device - use a fast blow fuse (as there is no normal in-rush or short-duration high current condition above max tube current draw).

Adding a 150V 5W zener across the fuse is an additional measure that can save the tube when the fuse has blown for other reasons that are not related to a 'bad tube'.

[pula58]

A fuse is a better defined protection device - use a fast blow fuse (as there is no normal in-rush or short-duration high current condition above max tube current draw).

Adding a 150V 5W zener across the fuse is an additional measure that can save the tube when the fuse has blown for other reasons that are not related to a 'bad tube'.

Thanks! Yeah, fast-blow fuse and maybe a parallel resistor.

To bring the thread back to my main question: When a power tube develops a short, what is shorting to what? And, where does the current go? In the case of a 6L6GC, do all the short currents end-up flowing out of pin 8 ?

I am trying to fuse an amp to protect the OT and PT from damage due to a bad power tube. I'd rather fuse the cathodes because the voltages are low there. I don't like the idea of using 250V rated fused for HT voltage of 450V.

Thanks to all so far!

[labb]

The one case I know for sure was an EL84. It shorted screen to grid.

[pula58]

A fuse is a better defined protection device - use a fast blow fuse (as there is no normal in-rush or short-duration high current condition above max tube current draw).

Adding a 150V 5W zener across the fuse is an additional measure that can save the tube when the fuse has blown for other reasons that are not related to a 'bad tube'.

Thanks! Yeah, fast-blow fuse and maybe a parallel resistor.

To bring the thread back to my main question: When a power tube develops a short, what is shorting to what? And, where does the current go? In the case of a 6L6GC, do all the short currents end-up flowing out of pin 8 ?

I am trying to fuse an amp to protect the OT and PT from damage due to a bad power tube. I'd rather fuse the cathodes because the voltages are low there. I don't like the idea of using 250V rated fused for HT voltage of 450V.

Thanks to all so far!

[labb]

The one case I know for sure was an EL84. It shorted screen to grid.

[trobbins]

Some discussion in links on the likely 'shorting' causes and how to manage them, as well as fusing protection.

http://dalmura.com.au/projects/Output%2 ... ection.pdf
http://dalmura.com.au/projects/Valve%20amp%20fusing.pdf

Ciao, Tim

[trobbins]

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